MAINT: Add docs-clean Pixi task

[VeckoTheGecko]

Description

It would be great to clean out artifacts (e.g., generated Parquet files) from the docs folder during building.

This PR does the following:

• Adds a docs-clean Pixi task which first cleans the docs folder then builds the docs
• Adds a docs-only-clean which just cleans the docs folder
• Updates the Pixi tasks to use the tasks used in the makefile definition
  • It would be great if we could just remove the makefile entirely, and leverage Pixi's task running for everything, however unfortunately the Pixi task runner isn't as flexible as we need it to be.

Checklist

• Closes None
• Tests added
• This PR targets the correct branch (main for normal development, v3-support for v3 support)

AI Disclosure

None

[erikvansebille]

pixi run docs-clean is a great idea!

[VeckoTheGecko]

Will merge once CI is fixed and this is rebased